Many oil-producing layers are found in stratified formations. For example, a mostly horizontal layer in an oil-bearing sedimentary formation may be bounded on the top and bottom by low permeability, non-oil producing, shale layers. Traditional vertical wells may produce oil from only a small portion of a stratified formation, draining a thin radial zone in oil-producing layers around the well.
The technology to drill and complete extended reach wells can increase the recovery of fluids from these stratified formations when compared to vertical wells. Extended reach wells, such as wells drilled out from offshore platforms or onshore "islands," are drilled and completed at an inclined angle to the vertical to follow the trend of the layer. The angle can be set so that a portion of the extended reach well is within a thin, nearly horizontal layer, or for a thicker layer, a less-than-horizontal incline path can slowly traverse or angle across the thicker layer. The nearly horizontal or angled portion is typically located below an initial (top), more-vertical portion drilled to reach the oil-producing layer.
Long, nearly horizontal or slanted wells can be more costly than a vertical well, but these extended reach wells may also be more productive for low permeability (i.e., "tight") reservoirs. The production is increased because of the greater surface area of the producing zone exposed to the wellbore, i.e., draining a larger portion of a tight productive layer.
However, problems maintaining the borehole portion drilled within a long thin layer, which can be composed of several producing sublayers, have been experienced. Even with current seismic survey data and imaging (accomplished prior to drilling), the extent, depth, and thickness of a thin oil-bearing layer is not always well known, especially over long distances. Even if the boundaries of the target layer are fairly well known, controlling the location of the borehole to follow a thin layer can present problems, especially when the face being drilled is several kilometers from the surface drilling location and the layer's thickness is measured in meters. In addition, the increased production from an extended reach well may not justify the increased cost of the extended reach well. Thus, achieving the goal of economic production from a new target zone, especially a small target zone, has not always been achieved.